Prescribed debt - Appaling service

This is the worst company ever! I had an MTN account about 5 years ago then lost my job and was unable to pay it.when I found employment again in 2021 I called MTN and was referred to Vericred collection who acknowledged that I was handed over and I set up a payment arrangment with them. Paid my account for almost a year only for the debit order to be stopped when i called to query why they said they have closed their book on their end. Few months later I'm contacted by nimble advising I owe almost 9k which is more than the amount I was initially advised by vericred. I queried It and sent bank statements showing that I have been paying for the past year and even sent the last statement I was sent by vericred but no one seems to know what is going on and when I call and email Im being cut off of someone says they will get back to me but no one does. Last week I received a letter of demand from them, I called and even sent documents on whatsapp but no one has a solution instead my credit is being affected by this. .....